Until now, to determine the presence or absence of foreign materials (steel materials of a different steel grade) in a stage such as before the shipping of steel materials having a substantially circular cross-section, there have been cases where steel grade determination by use of a fluorescent X-ray analysis method is conducted. In such a fluorescent X-ray analysis method, analysis is conducted by irradiating any fixed position of a steel material with X-rays for a predetermined time period.
In steel grade determination of a steel material, while there is a method to conduct fluorescent X-ray analysis on the outer peripheral surface of the steel material, in the case of a steel material having a surface layer oxide scale on the outer peripheral surface, such method has a problem in that the variation of analysis values among analysis positions increases since there are some elements that are unevenly distributed in the surface layer oxide scale. In particular, since Cr, Cu, and Ni are unevenly distributed in a surface layer oxide scale, the variation of analysis values among analysis positions will increase in steel pipes containing not less than 0.3 mass % of any element of Cr, Cu and Ni.
When the determination on whether or not a steel material to be subjected to steel grade determination is of any steel grade (hereafter, referred to as an “arbitrary steel grade”) is performed based on whether or not an analysis value of the steel material to be subjected to steel grade determination falls into a reference range which is predetermined in consideration of the composition range according to the manufacturing standard of the arbitrary steel grade and the variation of measurement; if the variation of analysis values among analysis positions is large, there is a risk that accurate steel grade determination cannot be performed since the analysis value may be out of the reference range of the arbitrary steel grade depending on analysis positions even if a steel material which is known in advance to be of the arbitrary steel grade is analyzed.
FIG. 1 is a diagram to show an example of analysis values of Cr when a fluorescent X-ray analysis of a steel pipe is performed at four points (positions of four directions of 0°, 90°, 180°, and 270° with assumption of the downward direction viewed from the pipe axis being 0°) in the pipe circumferential direction on the outer peripheral surface. While the analysis value of the molten steel of the steel material before casting is 1.04 mass %, the analysis values of the four points are 1.03 mass %, 1.18 mass %, 1.27 mass %, and 1.11 mass % showing a large variation.
On the other hand, when the surface oxide scale of this steel pipe is removed, it is confirmed that the variation of analysis values of Cr in the pipe circumferential direction is small. Therefore, the variation of the analysis values of Cr at four points in the pipe circumferential direction before removing the surface oxide scale is considered to be due to the surface oxide scale.
Accordingly, although it is conceivable to perform fluorescent X-ray analysis at multiple positions in the pipe circumferential direction and to use an average value of analysis values of respective positions to determine the steel grade of a steel material having a substantially circular cross-section, a problem exists in that it takes much time and effort to perform the fluorescent X-ray analysis at multiple positions.
Moreover, although a method is also conceivable in which the fluorescent X-ray analysis is performed after removing the surface oxide scale with a grinder etc. to eliminate the variation of analysis value due to the surface oxide scale, a problem also exists in that removing the surface oxide scale with a grinder requires time and effort.
Further, since a steel material having a substantially circular cross-section is subjected to end-face cutting for length adjustment and bevel processing after heat treatment, also conceivable is a method which performs fluorescent X-ray analysis on an end face of the steel material where there is no effect of surface oxide scale.
However, when fluorescent X-ray analysis is to be performed on an end face, the area which can be irradiated with fluorescent X-rays may become small depending on the diameter of steel material and the wall thickness of pipe when the steel material is a pipe, thereby making the analysis difficult.
Moreover, in a place where the steel material is transported in the longitudinal direction, it is difficult to install a fluorescent X-ray analyzer since if the fluorescent X-ray analyzer is installed in the transportation path of the steel material, there is a risk that the steel material collides with the fluorescent X-ray analyzer.
In particular, when fluorescent X-ray analysis is to be performed on an end face of the steel material in an automated manner in a manufacturing line, it is difficult to irradiate the end face of the steel material with fluorescent X-rays.
Further, as a method for determining a steel grade of a steel material, there is known a determination method described in Patent Literature 1; however, this determination method cannot solve the problem that steel grade determination cannot be performed easily and accurately in the fluorescent X-ray analysis of a steel material having a substantially circular cross-section.